One of the major problems in OFDM modulations is high Peak-to-Average Power Ratio (PAPR) of transmitted OFDM signals. Therefore, the OFDM receiver's detection efficiency is very sensitive to the nonlinear devices used in its signal processing loop, such as Digital-to-Analog Converter (DAC) and High Power Amplifier (HPA), which may severely impair system performance due to induced spectral regrowth and detection efficiency degradation.
For example, most radio systems employ the HPA in the transmitter to obtain sufficient transmits power and the HPA is usually operated at or near the saturation region to achieve the maximum output power efficiency, and thus the memory-less nonlinear distortion due to high PAPR of the input signals will be introduced into the communication channels.
If the HPA is not operated in linear region with large power back-off, it is impossible to keep the out-of-band power below the specified limits. This situation leads to very inefficient amplification and expensive transmitters.
When a HPA have a high dynamic range, it exhibits poor power efficiency. It has been shown that PAPR reduction can significantly save the power, in which the net power saving is directly proportional to the desired average output power and it is highly dependent upon the clipping probability level. Suppose that an ideal linear model for HPA, where linear amplification is achieved up to the saturation point, and thus we obtain:
  η  =      0.5    PAPR  
To illustrate the power inefficiency of a HPA in terms of the PAPR, we give an example of OFDM signals with 256 subcarriers and its CCDF has been shown in FIG. 6.
In order to guarantee that probability of the clipped OFDM frames is less than 0.01%, we need to apply an input backoff (IBO) equivalent to the PAPR at the 10−4 probability level, i.e. (25.235), referring to the FIG. 6, and thus the efficiency of HPA becomes ˜1.98
Therefore, so low efficiency is a strong motivation to reduce the PAPR in OFDM systems.
As shown in FIG. 7, different curves of the CCDF have been given for random original OFDM symbols generated and different PAPR reduction schemes. From FIG. 6, it is very clear that all schemes can reduce the PAPR largely in OFDM system. However, their performances of the PAPR reduction are different.
For example, when the PAPRs are 2.6 dB, 4.5 dB, 6.6 dB, 6.8 dB, 6.9 dB and 11.7 dB for the exponential companding, error companding, PTS, TR, clipping scheme and original OFDM signals, respectively. Obviously, the signals compounded by the nonlinear companding transform with exponential function can reduce the PAPR largest and the PAPR reduction of the clipping scheme is the smallest among these typical methods. Although clipping scheme can improve its performance of the PAPR reduction through reducing its preset clipping level A.
However, the performance of the BER will be degraded largely when its preset clipping level is reduced.
FIG. 7 depicts the performance of BER versus SNR of actual OFDM signals with PAPR reduction based on different schemes over the AWGN channel, in which the typical HPA of the Solid State Power Amplifier (S SPA) has been considered. Note that SSPA produces no phase distortion and only the AM/AM conversion. In FIG. 7, the performance bounds are obtained by ignoring the effect of SSPA and directly transmitting the original OFDM signals through the AWGN channels. Generally speaking, the performances of the BER with different PAPR reduction schemes have some degradation from FIG. 7. Specifically, to achieve a BER of the minimum required SNR is 13.8 dB (performance bound).
There is a growing need to provide efficient systems, computer readable media and method for generating modified OFDM symbols that will comply with reduced peak to average ration requirements.